Noise is produced by objects in vibration, and reducing noise to create quiet environments has been a goal in human civilizations. Noise are produced by many sources, including, but not limited to, vehicles on the road, airplanes (in flying, taking-off or landing such as on runway, aircraft carrier decks), machining processes (machining on lathe, grinding, sand blasting, etc.), appliance (washing machine, refrigerator, dishwasher, etc.), gunshots, and explosions to name a few. Exposure to noise can induce hearing loss. A long-time exposure to medium intensity sound in situations such as sand blaster operation, or a short time exposure to high intensity sound such as those induced by blast can induce irreversible hearing loss.
Significant endeavors have been made by materials science and acoustics experts for innovation of materials or hearing protection devices that produce noise attenuation, using single layer or multiple layers of materials in the design. In the current art, the primary sound insulation materials are microporous materials, such as polyurethane foams, the pores of which have dimensions on the order of tens of microns or higher. Noise is attenuated when sound travels through a torturous path, or through the walls with irregular shapes. The walls, however, are made of homogeneous materials with geometry changing continuously, so that the sound wave can travel through without much resistance. In addition, the pores are large so that sound can travel through the pores without much resistance. Since the entire foam is made of homogeneous skeletal material (FIG. 1), in which sound wave can travel freely from one location to another, the primary mechanism for attenuation of sound is the irregular geometry in porous walls that induce sound reflection, scattering, and diffraction, and its resulting torturous path, which is very limited. As a result, the sound attenuation loss when a sound transmits through a unit thickness of such materials is small, on the order of less than 5 dB/cm.
Traditional acoustic materials provide much lower sound transmission loss. In many critical applications, such as aerospace, aviation, defense applications, power plant, medical devices etc. noise control is a very challenging task. In order to achieve high transmission loss, either bulky or heavy materials have to be applied, even though, the sound transmission loss cannot be achieved as high as those critical applications require.
It would therefore be desirable to develop materials having better sound attenuation properties than those exhibited by currently available options. It is an object of the invention to provide a nanostructured material, which exhibits superior acoustic attenuation properties relative to prior art compositions. This invention provides low-cost, lightweight, thin-materials (panels or required curved shapes) which can provide high sound transmission loss as high as 40-70 dB or greater. Combining their excellent thermo-insulation and mechanical properties, this invention can be applied to many critical applications where the high sound transmission loss is required.